Geophex, Ltd. has developed a vehicle-towed, electromagnetic (EM) gradiometer array, called GEM-5B, that may be used for geologic mapping, as well as detecting/characterizing manmade underground structures like clandestine tunnels. It can also passively detect loaded powerlines by measuring background EM spectra that may emanate from lighting, ventilation, and water-pumping related to tunnel operation.

The GEM-5B array is based in principle on Geophex's GEM-5 gradiometer but with increased transmitter dipole moment for deep penetration and improved signal-to-noise ratio in a broad bandwidth. The GEM-5 array is designed for detecting shallow targets such as landmines, unexploded ordnance, and utility lines that are typically buried about 2 meters or less.

The array produces real time graphic data display inside the towing vehicle. The array measures inphase (I) and quadrature (Q) responses at 8-12 programmable frequencies. From these raw data, one can derive apparent conductivity at each frequency and magnetic susceptibility at the lowest specified frequency. The sensor can also measure passive emissions at powerline frequency and its harmonics. The array will have the following properties:

• Large transmitter dipole moment about 3,000 A.m2 at 570 Hz,
• Broadband operation, typically 10 frequencies between 90 Hz - 96kHz,
• Simultaneous operation of all receiver channels,
• Immunity to noise induced by sensor motion and to ambient environmental EM noise,
• Detecting passive powerline emission from power usage in the tunnel, and
• Increased SNR owing to multiple receivers with the noise immunities.